My version of the AP takes place in Greyhawk, as we had quite a lot of fun there when we started through the original D&D 3E adventure path (never got to finish Ashardalon, though).
I've set everything in Ratik, which provided a pretty great fit for the first two adventures. Magnimar became Marner, the Varisians are semi-nomadic Flan, Myrhiss became the patron god of Sandpoint. I've also got more freedom by setting it in 150 years after the Living Greyhawk Gazetter, rewriting and resizing the lands a bit.
But now we're heading out of Marner/Magnimar into the mountains, and I need to expand on my campaign background a bit, which is why I'm asking people more experience in all things Flanaess about some pointers.
One issue that I have, is getting Thassilon into the picture. Greyhawk seems a pretty young civilization, where the big migration just happened a few years ago, and most of adventuring lands were pretty unspoilt back then. The Ur-Flan don't seem to be civilized enough. Then again, I guess that I'm missing some adventures in Greyhawk, maybe some ancient Gygaxian tome already included something that I could tie in with Karzoug. Whether it's 3000 or 10000 years in the past hardly matters.
Also, the Hook Mountain location: I always found the 450 miles that the party had to travel from Magnimar to Turtleback Ferry to be a bit too much for the sphere of influence of a city state, and in Greyhawk it's definitely out. I was thinking about a mountain range in the Griff mountains, probably with some added incentive that makes it worth the while for Marner (mines?). Possibly some intervention from the Theocracy of the Pale on the other side.
I notice that there are some similarities between Golarion and Greyhawk, so I hope that someone can point out a few of them for me to mine…